docs: def cached_value(self, source_file, configuration):
          """
          Attempt to lookup the cached declarations for the given file and
          configuration.
  
          Returns None if declaration not found or signature check fails.
  
          """
  
          key = record_t.create_key(source_file, configuration)
          if key not in self.__cache:
              return None
          record = self.__cache[key]
          if self.__is_valid_signature(record):
              record.was_hit = True  # Record cache hit
              return record.declarations
  
          # some file has been changed
          del self.__cache[key]
          return None